U.S. patent number 5,170,493 [Application Number 07/223,911] was granted by the patent office on 1992-12-08 for combined low frequency receive and high frequency transceive antenna system and method.
This patent grant is currently assigned to IIMorrow, Inc.. Invention is credited to Stephen A. Roth.
United States Patent |
5,170,493 |
Roth |
December 8, 1992 |
Combined low frequency receive and high frequency transceive
antenna system and method
Abstract
A dual function antenna system and method. A system is provided
for using a single antenna for both the reception of low frequency
navigational signals, such as LORAN-C, and high frequency
communications signals in the VHF region. The antenna is at a
remote location from a low frequency receiver and a high frequency
transceiver. Both the low frequency and the high frequency signals
are coupled to the receiver and transceiver, respectively, over the
same transmission line at one end thereof. A preamplifier is
provided at the location of the antenna to increase the strength of
the low frequency signal applied to the transmission line. A high
pass filter is provided to couple the antenna and the transmission
line and prevent feedback from the output of the preamplifier from
reaching its input. The other end of the transmission line is
coupled to the high frequency transceiver by a high pass filter,
and to the low frequency receiver by a low pass filter. The
preamplifier is supplied with DC power over the transmission line,
and includes a current regulator so that the preamplifier may be
used with a wide range of supply voltages.
Inventors: |
Roth; Stephen A. (Aumsville,
OR) |
Assignee: |
IIMorrow, Inc. (Salem,
OR)
|
Family
ID: |
22838489 |
Appl.
No.: |
07/223,911 |
Filed: |
July 25, 1988 |
Current U.S.
Class: |
455/82; 333/126;
333/132; 455/282; 455/291; 455/83 |
Current CPC
Class: |
H04B
1/18 (20130101); H04B 1/48 (20130101); H01Q
5/50 (20150115) |
Current International
Class: |
H01Q
5/00 (20060101); H04B 1/18 (20060101); H04B
1/48 (20060101); H04B 1/44 (20060101); H04B
001/46 (); H04B 001/18 () |
Field of
Search: |
;455/78,80,82-83,142-143,282,291 ;333/126,129,132,134 ;343/858,861
;342/57-58,388,389,55 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Eisenzopf; Reinhard J.
Assistant Examiner: Faile; Andrew
Attorney, Agent or Firm: William A. Birdwell &
Associates
Claims
What is claimed is:
1. A system for coupling a radio antenna to a high frequency
transmitter and a low frequency receiver over the same first
transmission line so that said antenna may be simultaneously used
to transmit high frequency signals and receive low frequency
signals, comprising:
(a) preamplifier means for connecting said antenna to said first
transmission line, amplifying low frequency signals present at said
antenna, and applying said low frequency signals, as amplified, to
said first transmission line, said preamplifier means having an
input port for receiving signals to be amplified and an output port
for outputting amplified signals; and
(b) high frequency path means for coupling high frequency signals
present on said first transmission line to said antenna without
coupling to said antenna said low frequency signals applied to said
first transmission line by said preamplifier so as to prevent
feedback of low frequency signals output at said output port of
said preamplifier to said input port thereof.
2. The system of claim 1, wherein said preamplifier means has an
input port for receiving signals to be amplified and an output port
for outputting amplified signals, said system further comprising
low frequency input network means, disposed between said antenna
and said input port of said preamplifier means, for substantially
matching the impedance of said antenna to the impedance of said
preamplifier means at said input port at the frequency of said low
frequency signals.
3. The system of claim 2, wherein said preamplifier means comprises
amplifier means, disposed between said input port and said output
port, for amplifying signals applied to said input port, and
current regulator means for supplying substantially constant
current DC power to said amplifier means.
4. The system of claim 3, wherein said preamplifier means further
comprises means for receiving DC power for said amplifier means at
said output port and applying said DC power to said amplifier means
through said current regulator means.
5. The system of claim 2, wherein said low frequency input network
means comprises band pass filter means for limiting the range of
frequencies which can be coupled from said antenna to said input
port of said preamplifier means to a predetermined range of
frequencies corresponding to said low frequency signals.
6. The system of claim 5, wherein said low frequency input network
means comprises a filter whose elements include the distributed
reactance of said antenna.
7. The system of claim 5, wherein said low frequency signals are
LORAN-C navigation signals and said low frequency input network
means has a band pass characteristic of approximately 100 kHz.+-.25
kHz.
8. The system of claim 1 further comprising low pass filter means,
connected to said first transmission line, for coupling said low
frequency signals, as amplified, from said first transmission line
to a low frequency receiver, and high pass filter means, connected
to said first transmission line, for coupling said high frequency
signals from said transmitter to said first transmission line.
9. A method for coupling a radio antenna to a high frequency
transmitter and a low frequency receiver over the same first
transmission line so that said antenna may be simultaneously used
to transmit high frequency signals and receive low frequency
signals, comprising:
(a) amplifying low frequency signals present at said antenna and
applying said low frequency signals, as amplified, to said first
transmission line; and
(b) coupling high frequency signals present on said first
transmission line to said antenna without coupling to said antenna
said amplified low frequency signals applied to said first
transmission line thereby preventing said amplified low frequency
signals from producing feedback and concommitant instability.
10. The method of claim 9, further comprising coupling high
frequency signals present on said first transmission line to said
antenna and vice versa without substantially attenuating low
frequency signals present on said antenna.
11. The method of claim 9 wherein said amplifying step includes
matching the impedance of said antenna to the impedance of said
first transmission line at the frequency of said low frequency
signals.
12. The method of claim 9, further comprising employing an
amplifier in said amplifying step and matching the impedance of
said antenna to the input impedance of said amplifier at the
frequency of said low frequency signals.
13. The method of claim 9, further comprising coupling high
frequency signals from said transmitter to said first transmission
line without substantially attenuating low frequency signals
present on said first transmission line.
14. The method of claim 9, further comprising coupling low
frequency signals present on said first transmission line to said
receiver, without coupling high frequency signals present on said
transmission line to said receiver.
15. A system for coupling a radio antenna to a high frequency
transmitter and a low frequency receiver over the same first
transmission line so that said antenna may be simultaneously used
to transmit high frequency signals and receive low frequency
signals, comprising:
(a) preamplifier means for connecting said antenna to said first
transmission line, amplifying low frequency signals present at said
antenna, and applying said low frequency signals, as amplified, to
said first transmission line, said preamplifier means having an
input port for receiving signals to be amplified and an output port
for outputting amplified signals;
(b) low frequency input network means, disposed between said
antenna and said input port of said preamplifier means, for
substantially matching the impedance of said antenna to the
impedance of said preamplifier means at said input port at the
frequency of said low frequency signals;
(c) low pass filter means, disposed between said output port of
said preamplifier means and said first transmission line, for
permitting said low frequency signals, as amplified, to be passed
from said preamplifier means to said first transmission line and DC
power to be passed from said first transmission line to said
preamplifier means, without allowing high frequency signals present
on said first transmission line to be passed from said first
transmission line to said output port of said preamplifier means;
and
(d) high frequency path means for coupling high frequency signals
present on said first transmission line to said antenna without
coupling to said antenna said low frequency signals applied to said
first transmission line.
16. A system for coupling a radio antenna to a high frequency
transmitter and a low frequency receiver over the same first
transmission line so that said antenna may be simultaneously used
to transmit high frequency signals and receive low frequency
signals, comprising:
(a) preamplifier means for connecting said antenna to said first
transmission line, amplifying low frequency signals present at said
antenna, and applying said low frequency signals, as amplified, to
said first transmission line; and
(b) high frequency path means for coupling high frequency signals
present on said first transmission line to said antenna without
coupling to said antenna said low frequency signals applied to said
first transmission line, said high frequency path means comprising
a band pass filter connected between said antenna and said first
transmission line, for limiting the range of frequencies which can
be coupled between said antenna and said first transmission line
via said high frequency path to a predetermined range of high
frequency signals.
17. The system of claim 16, wherein said band pass filter is
coupled to said antenna through a first series capacitor.
18. The system of claim 17, wherein said first series capacitor has
a capacitance of substantially about 3.9 picofarads or less.
19. The system of claim 16, wherein said band pass filter is
coupled to said first transmission line through a second series
capacitor.
20. The system of claim 16, wherein said high frequency signals are
VHF communication signals and said band pass filter permits signals
of approximately 150 MHz.+-.2.5 MHz to pass therethrough with a
good impedance match to said antenna and to said first transmission
line.
21. A system for coupling a radio antenna to a high frequency
transmitter and a low frequency receiver over the same first
transmission line so that said antenna may be simultaneously used
to transmit high frequency signals and receive low frequency
signals, comprising:
(a) preamplifier means for connecting said antenna to said first
transmission line, amplifying low frequency signals present at said
antenna, and applying said low frequency signals, as amplified, to
said first transmission line, said preamplifier means including
means for receiving DC power from said first transmission line;
(b) high frequency path means for coupling high frequency signals
present on said first transmission line to said antenna without
coupling to said antenna said low frequency signals applied to said
first transmission line;
(c) low pass filter means, connected to said first transmission
line, for coupling said low frequency signals, as amplified, from
said first transmission line to a low frequency receiver, said low
pass filter means including means for passing DC power for said
preamplifier means to said first transmission line; and
(d) high pass filter means, connected to said first transmission
line, for coupling said high frequency signals from said
transmitter to said first transmission line.
22. A system for coupling a radio antenna to a high frequency
transmitter and a low frequency receiver over the same first
transmission line so that said antenna may be simultaneously used
to transmit high frequency signals and receive low frequency
signals, comprising:
(a) preamplifier means for connecting said antenna to said first
transmission line, amplifying low frequency signals present at said
antenna, and applying said low frequency signals, as amplified, to
said first transmission line;
(b) high frequency path means for coupling high frequency signals
present on said first transmission line to said antenna without
coupling to said antenna said low frequency signals applied to said
first transmission line; and
(c) a second, relatively short transmission line electrically
disposed between said antenna and said high frequency path means,
said high frequency path means comprising a network for matching
the impedance of said second transmission line to the impedance of
said first transmission line at the frequency of said high
frequency signals.
23. The system of claim 22, wherein said preamplifier means has an
input port for receiving signals to be amplified and an output port
for outputting amplified signals, said system further comprising
low frequency input network means, disposed between said second
transmission line and said input port of said preamplifier means,
for substantially matching the impedance of said antenna to the
impedance of said preamplifier means at said input port at the
frequency of said low frequency signals, said low frequency input
network means having an input choke means for substantially
blocking said high frequency signals.
24. A method for coupling a radio antenna to a high frequency
transmitter and a low frequency receiver over the same first
transmission line so that said antenna may be simultaneously used
to transmit high frequency signals and receive low frequency
signals, comprising:
(a) employing an amplifier to amplify low frequency signals present
at said antenna and applying said low frequency signals, as
amplified, to said first transmission line;
(b) coupling high frequency signals present on said first
transmission line to said antenna without coupling to said antenna
said low frequency signals applied to said first transmission line;
and
(c) filtering signals present at said antenna before applying them
to said amplifier so as to permit low frequency signals present at
said antenna to pass to the input of said amplifier while
preventing high frequency signal present at said antenna from
passing to the input of said amplifier.
25. The method of claim 24, wherein said filtering includes
substantially preventing frequencies above and below a
predetermined range of frequencies from passing to the input of
said amplifier.
26. A method for coupling a radio antenna to a high frequency
transmitter and a low frequency receiver over the same first
transmission line so that said antenna may be simultaneously used
to transmit high frequency signals and receive low frequency
signals, comprising:
(a) employing an amplifier to amplify low frequency signals present
at said antenna and applying said low frequency signals, as
amplifier, to said first transmission line;
(b) coupling high frequency signals present on said first
transmission line to said antenna without coupling to said antenna
said low frequency signals applied to said first transmission line;
and
(c) filtering the output of said amplifier so as to permit said low
frequency signals, as amplified, to be coupled to said first
transmission line but to prohibit high frequency signals present on
said first transmission line from being applied to said
amplifier.
27. A method for coupling a radio antenna to a high frequency
transmitter and a low frequency receiver over the same first
transmission line so that said antenna may be simultaneously used
to transmit high frequency signals and receive low frequency
signals, comprising:
(a) employing an amplifier to amplify low frequency signals present
at said antenna and applying said low frequency signals, as
amplified to said first transmission line;
(b) coupling high frequency signals present on said first
transmission line to said antenna without coupling to said antenna
said low frequency signals applied to said first transmission line;
and
(c) regulating the current used by said amplifier so a to permit
said amplifier to operate over a wide range of supply voltages.
28. The method of claim 27, wherein current is supplied to said
amplifier over said first transmission line.
29. A method for coupling a radio antenna to a high frequency
transmitter and a low frequency receiver over the same first
transmission line so that said antenna may be simultaneously used
to transmit high frequency signals and receive low frequency
signals, comprising:
(a) amplifying low frequency signals present at said antenna and
applying said low frequency signals, as amplifier, to said first
transmission line;
(b) coupling high frequency signals present on said first
transmission line to said antenna without coupling to said antenna
said low frequency signals applied to said first transmission
line;
(c) connecting a relatively short second transmission line to said
antenna; and
(d) matching the impedance of said second transmission line to the
impedance of said first transmission line at the frequency of said
low frequency signals.
30. A dual-purpose antenna system, comprising:
(a) a radio antenna having relatively low impedance at high
frequencies and relatively high impedance at low frequencies;
(b) first coupling means for coupling said antenna to a first
transmission line at said low frequencies, said first coupling
means including amplifier means for amplifying signals at said low
frequencies and means for receiving DC power from said first
transmission line and applying it to said amplifier means;
(c) second coupling means for coupling said antenna to said first
transmission line at said high frequencies;
(d) third coupling means for coupling said first transmission line
to a low frequency radio receiver at said low frequencies, said
third coupling means including means for passing said DC power to
said transmission line; and
(e) fourth coupling means for coupling said first transmission line
to a high frequency radio transmitter at said high frequencies.
31. A dual-purpose antenna system, comprising:
(a) a radio antenna having relatively low impedance at high
frequencies and relatively high impedance at low frequencies;
(b) first coupling means for coupling said antenna to a first
transmission line at said low frequencies;
(c) second coupling means for coupling said antenna to said first
transmission line at said high frequencies;
(d) third coupling means for coupling said first transmission line
to a low frequency radio receiver at said low frequencies;
(e) fourth coupling means for coupling said first transmission line
to a high frequency transmitter; and
(f) second transmission line disposed between said antenna and said
first and second coupling means.
32. A system for coupling a radio antenna to a high frequency
receiver and low frequency receiver over the same first
transmission line so that said antenna may be simultaneously used
to receive high frequency signals and receive low frequency
signals, comprising:
(a) preamplifier means for connecting said antenna to said first
transmission line, amplifying low frequency signals present at said
antenna, and applying said low frequency signals, as amplified, to
said first transmission line, said preamplifier means having an
input port for receiving signals to be amplified and an output port
for outputting amplified signals; and
(b) high frequency path means for coupling high frequency signals
present at said antenna to said first transmission line without
coupling to said antenna said low frequency signals applied to said
first transmission line by said preamplifier so as to prevent
feedback of low frequency signals output at said output port of
said preamplifier to said input port thereof.
33. A method for coupling a radio antenna to a high frequency
receiver and a low frequency receiver over the same first
transmission line so that said antenna may be simultaneously used
to receive high frequency signals and receive low frequency
signals, comprising:
(a) amplifying low frequency signals present at said antenna and
applying said low frequency signals, as amplified, to said first
transmission line; and
(b) coupling high frequency signals present at said antenna to said
first transmission line without coupling to said antenna said
amplified low frequency signals applied to said first transmission
line, thereby preventing said amplified low frequency signals from
producing feedback and concomitant instability.
34. A system for coupling a radio antenna to a high frequency
receiver and a low frequency receiver over the same first
transmission line so that said antenna may be simultaneously used
to receive high frequency signals and receive low frequency
signals, comprising:
(a) preamplifier means for connecting said antenna to said first
transmission line, amplifying low frequency signals present at said
antenna, and applying said low frequency signals, as amplified, to
said first transmission line; and
(b) high frequency path means for coupling high frequency signals
present at said antenna to said first transmission line to said
antenna without coupling to said antenna said low frequency signals
applied to said first transmission line, said high frequency path
means comprising a band pass filter connected between said antenna
and said first transmission line, for limiting the range of
frequencies which can be coupled between said antenna and said
first transmission line via said high frequency path to a
predetermined range of high frequency signals.
35. The system of claim 34, wherein said band pass filter is
coupled to said antenna through a first series capacitor.
36. The system of claim 35, wherein said first series capacitor has
a capacitance of substantially about 3.9 picofarads or less.
37. The system of claim 34, wherein said band pass filter is
coupled to said first transmission line through a second series
capacitor.
38. The system of claim 34, wherein said high frequency signals are
VHF communication signals and said band pass filter permits signals
of approximately 150 MHz.+-.2.5 MHz to pass therethrough with a
good impedance match to said antenna and to said first transmission
line.
39. A system for coupling a radio antenna to a high frequency
receiver and a low frequency receiver over the same first
transmission line so that said antenna may be simultaneously used
to receive high frequency signals and receive low frequency
signals, comprising:
(a) preamplifier means for connecting said antenna to said first
transmission line, amplifying low frequency signals present at said
antenna, and applying said low frequency signals, as amplified, to
said first transmission line;
(b) high frequency path means for coupling high frequency signals
present at said antenna to said first transmission line without
coupling to said antenna said low frequency signals applied to said
first transmission line; and
(c) a second, relatively short transmission line electrically
disposed between said antenna and said high frequency path means,
said high frequency path means comprising a network for matching
the impedance of said second transmission line to the impedance of
said first transmission line at the frequency of said high
frequency signals.
40. The system of claim 39, wherein said preamplifier means has an
input port for receiving signals to be amplified and an output port
for outputting amplified signals, said system further comprising
low frequency input network means, disposed between said second
transmission line and said input port of said preamplifier means,
for substantially matching the impedance of said antenna to the
impedance of said preamplifier means at said input port at the
frequency of said low frequency signals, said low frequency input
network means having an input choke means for substantially
blocking said high frequency signals.
41. A method for coupling a radio antenna to a high frequency
receiver and a low frequency receiver over the same first
transmission line so that said antenna may be simultaneously used
to receive high frequency signals and to receive low frequency
signals, comprising:
(a) employing an amplifier to amplify low frequency signals present
at said antenna and applying said low frequency signals, as
amplified, to said first transmission line;
(b) coupling high frequency signals present at said antenna to said
first transmission line without coupling to said antenna said low
frequency signals applied to said first transmission line; and
(c) filtering signals present at said antenna before applying them
to said amplifier so as to permit low frequency signals present at
said antenna to pass to the input of said amplifier while
preventing high frequency signals present at said antenna from
passing to the input of said amplifier.
42. The method of claim 41, wherein said filtering includes
substantially preventing frequencies above and below a
predetermined range of frequencies from passing to the input of
said amplifier.
43. A method for coupling a radio antenna to a high frequency
receiver and a low frequency receiver over the same first
transmission line so that said antenna may be simultaneously used
to receive high frequency signals and receive low frequency
signals, comprising:
(a) employing an amplifier to amplify low frequency signals present
at said antenna and applying said low frequency signals, as
amplified, to said first transmission line;
(b) coupling high frequency signals present at said antenna to said
first transmission line without coupling to said antenna said low
frequency signals applied to said first transmission line; and
(c) regulating the current used by said amplifier so as to permit
said amplifier to operate over a wide range of supply voltages.
44. The method of claim 43, wherein current is supplied to said
amplifier over said first transmission line.
45. A method for coupling a radio antenna to a high frequency
receiver and a low frequency receiver over the same first
transmission line so that said antenna may be simultaneously used
to receive high frequency signals and to receive low frequency
signals, comprising:
(a) amplifying low frequency signals present at said antenna and
applying said low frequency signals, as amplified, to said first
transmission line;
(b) coupling high frequency signals present at said antenna to said
first transmission line without coupling to said antenna said low
frequency signals applied to said first transmission line;
(c) connecting a relatively short second transmission line to said
antenna;
(d) matching the impedance of said second transmission line to the
impedance of said first transmission line at the frequency of said
low frequency signals.
Description
BACKGROUND OF THE INVENTION
This invention relates to radio antenna systems and methods,
particularly antenna systems and methods for enabling one antenna
to transmit a high frequency radio signal and simultaneously
receive a low frequency radio signal.
In vehicles, such as automobiles or airplanes, it is often
desirable or necessary to employ both radio navigation and radio
communications equipment. A common type of radio navigation is a
system known as LORAN, which works by receiving at a vehicle
encoded low frequency radio signals transmitted from fixed
locations, based upon which the vehicle's position can be computed.
LORAN-C is the LORAN system currently in predominate use, and its
signals are broadcast at a carrier frequency of 100 kilohertz
("kHz").
In contrast, two-way radio communications with land and air
vehicles typically employ VHF frequencies in the VHF band. VHF
carrier frequencies range from about 30 megahertz ("MHz") up to
about 300 MHz. Typically, a vehicle would carry a VHF radio
frequency transceiver. It is often desirable for the vehicle
operator simultaneously to be receiving LORAN signals for
navigation and to be transmitting or receiving VHF signals for
communication.
Notwithstanding the considerable difference in frequency of LORAN
and VHF signals, it is often desirable to employ the same antenna
on a vehicle both to receive low frequency LORAN signals and to
transmit or receive VHF communication signals. Moreover, it is
ordinarily desirable that the antenna be relatively short, on the
order of about 20 inches. In the case of an automobile one reason
is to minimize the obtrusiveness of the antenna. This may be
because the user does not want an unsightly antenna on the car,
because some degree of concealment of the antenna may be important
to law enforcement agencies, or simply to avoid attracting the
attention of persons who may be tempted to steal expensive
electronic equipment within the vehicle. In the case of an
airplane, the use of a single, short antenna contributes to the
operation of the airplane by minimizing the number and size of
protrusions from the airplane so as to decrease drag. It is also
important to minimize the number of items that would otherwise
contribute unnecessary weight to the aircraft. Thence, it is
desirable to use a single, relatively short antenna both to receive
low frequency navigation signals, such as LORAN, and simultaneously
transmit or receive high frequency, typically VHF, radio
communications.
There are some serious obstacles to achieving the foregoing
objective. In many vehicles, particularly in aircraft, the antenna
must be located some distance from the LORAN receiver and from a
VHF transceiver. For example, the antenna would typically be
located on the top of an airplane fuselage, while the LORAN
navigation equipment and VHF transceiver would be located in the
cockpit. Ordinarily, this would require two separate transmission
lines between the antenna and the radio electronics, which is not
only inconvenient but contributes undesired weight to the aircraft.
Another significant problem is that, while VHF signals have
wavelengths not too much greater than such a short antenna as a
result of which the antenna has a relatively low impedance at VHF
frequencies, the wavelength of LORAN signals is much longer than
the antenna, so that the antenna has a very high impedance at low,
LORAN signal frequencies. This means that it is relatively easy to
match the antenna to the VHF transceiver for maximum transfer of
power, but that it is difficult to do so for a LORAN signal.
Consequently, use of a short antenna to receive LORAN signals will
ordinarily result in a low signal-to-noise ratio.
Use of a single antenna both for transmitting VHF communications
and receiving LORAN must also be accomplished such that the
transmitted VHF signal does not damage, or even overload, the input
of the LORAN receiver. Also, such an antenna should be versatile,
so that it can be used with a variety of electronic equipment.
A number of systems for employing the same antenna to both receive
relatively low frequency radio signals and to transmit or receive
relatively high frequency radio signals have been developed. Such
systems are disclosed, for example, in Tanner, et al. U.S. Pat. No.
4,268,805; Elliott, U.S. Pat. No. 4,095,229; and Tyrey, U.S. Pat.
No. 4,037,177. However, in each of these either the radio equipment
would have to be located relatively close to the antenna, or more
than one transmission line would be required to connect low and
high frequency matching networks to the radio equipment. Moreover,
none of these systems deals with the simultaneous use of LORAN
reception and VHF transmission. Powell, et al. U.S. Pat. No.
3,812,494 discloses a system for receiving and transmitting Doppler
frequencies and telemetering frequencies simultaneously over the
same antenna, as distinguished from low frequency LORAN and VHF
frequencies, and employs an impedance matching network in close
proximity with the antenna itself. Duncan, Jr., et al. U.S. Pat.
No. 3,217,273 discloses a system for coupling several transmitters
and receivers to a single antenna, while avoiding the radiation of
spurious signals resulting from intermodulation, and avoiding
swamping the receivers. In addition, LORAN antenna systems which
employ a current regulated preamplifier located at a remote
antenna, and powered by DC current provided over the
antenna-to-receiver transmission line, to prevent reduction of the
signal-to-noise ratio between the antenna and the LORAN receiver
have previously been used. However, none of these systems solves
the aforementioned problems associated with employing a relatively
short antenna disposed some distance away from LORAN and VHF
communications equipment both to receive LORAN signals and transmit
or receive VHF communications simultaneously.
SUMMARY OF THE INVENTION
The present invention provides a dual-purpose antenna system which
overcomes the aforementioned obstacles. A relatively short antenna
is coupled to one end of a transmission line through a split
coupling circuit proximate the antenna. One path of the split
circuit is for receiving low frequency, LORAN signals and coupling
them to the transmission line. It includes a preamplifier for
amplifying the received signals before they are applied to the
transmission line to prevent reduction of the signal-to-noise ratio
between the antenna and a LORAN receiver, and a low frequency input
band pass filter. This filter both blocks signals present at the
antenna outside the necessary band pass from reaching the
preamplifier and matches the impedance of the short antenna to the
input impedance of the preamplifier at about 100 kHz. The second
path of the split circuit couples VHF signals on the transmission
line to the antenna and vice versa. It employs a band pass filter
which allows VHF signals of about 150 MHz to pass, but blocks low
frequency LORAN signals, and may also perform an impedance matching
function.
The system also includes, at the other end of the transmission
line, a low pass filter for coupling the LORAN signals from the
transmission line to the LORAN receiver, and a high pass filter for
coupling VHF signals from the transmission line to a VHF
transceiver, and vice versa. Thus, the low frequency and high
frequency signal paths split at the antenna, are recombined at one
end of a transmission line, and split at the other end of the
transmission line for connection to the radio equipment.
The preamplifier at the antenna is powered by direct current ("DC")
supplied over the transmission line. The preamplifier is provided
with a current regulation circuit so that the antenna can be used
with various LORAN equipment that apply different voltages.
The input band pass filter of the preamplifier and the VHF band
pass filter in the split coupling circuit are both designed to
minimize shunt capacitance at the antenna, so as to minimize
attenuation of the LORAN signal. Advantage is taken of distributed
reactance of the antenna and of the circuit elements in these
filters to achieve the desired impedance matching and filtering
characteristics with minimal attenuation of LORAN signals.
Accordingly, it is a principal objective of the present invention
to provide a novel and improved antenna system and method whereby a
single, relatively short antenna can be used both to receive low
frequency radio signals and to transmit high frequency radio
signals simultaneously.
It is another objective of the present invention to provide a
system and method for coupling a single radio antenna to a high
frequency radio transmitter and a low frequency radio receiver over
the same transmission line so that high frequency radio signals may
be transmitted and low frequency radio signals may be received
simultaneously.
It is another objective of the present invention to provide such a
coupling system and method which includes preamplification of the
low frequency signals prior to their application to the
transmission line so as to prevent further reduction of their
already low signal-to-noise ratio.
It is another objective of the present invention to provide such a
coupling system and method wherein the impedance of the antenna at
low frequencies is matched through one signal path to the
transmission line and the impedance of the antenna at high
frequencies is matched through another signal path to the
transmission line.
It is a further objective of the present invention to provide for
an antenna system and method wherein a relatively short antenna at
a location remote from a LORAN receiver and a VHF transceiver is
coupled by one transmission line to both the LORAN receiver and the
VHF receiver.
The foregoing and other objectives, features, and advantages of the
present invention will be more readily understood upon
consideration of the following detailed description of the
invention, taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a preferred embodiment of an antenna
system according to the present invention.
FIG. 2 is a schematic diagram of a preferred embodiment of an
antenna system according to the present invention.
FIG. 3 is a schematic diagram of an equivalent circuit for a low
frequency band pass filter according to the present invention.
FIG. 4 is a schematic diagram of an equivalent circuit for a high
frequency band pass filter according to the present invention.
FIG. 5 is a schematic diagram of an alternative embodiment of an
antenna system according to the present invention.
FIG. 6 is a schematic diagram of an exemplary equivalent circuit
for an antenna having a short length of transmission line connected
thereto.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In FIG. 1, which shows a block diagram of the preferred embodiment
of the invention, a relatively short antenna 10 is coupled to a low
frequency receiver 12 and a high frequency transceiver 14. The
antenna may be used simultaneously to receive low frequency radio
signals and transmit or receive high frequency radio signals. The
system is particularly adapted to receive navigational signals,
such as LORAN-C, which are transmitted at about 100 kHz.+-.10 kHz,
and to simultaneously transmit or receive VHF communication
signals, transmitted at about 150 MHz.+-.2.5 MHz. However, it
should be recognized that the invention might be employed in other
applications as well.
The antenna 10 would typically be mounted on a vehicle, such as an
automobile or an aircraft, in which a VHF transceiver is employed
for communications and a LORAN receiver is used for navigation. The
antenna would preferably be about 20 inches in length. At that
length, the antenna would be relatively unobtrusive, can be made
relatively lightweight, would produce little air drag, and would
present an essentially resistive termination impedance of about 50
ohms at 150 MHz. On the other hand, it would present an impedance
of about 200 kohms, with a significant capacitive reactance
component, at 100 kHz.
The antenna 10 would ordinarily be mounted on the outside of a
vehicle at some distance from the receiver 12 and the transceiver
14, so a transmission line 16, typically a length of 50 ohm coaxial
cable, is employed to connect the antenna to the receiver and
transceiver. The low frequency LORAN-C signals and the VHF
communications signals are coupled to and from the antenna over the
same transmission line 16.
Since the signal strength of the LORAN signals is relatively low to
begin with, and the impedance of the antenna is very high at 100
kHz, the amplitude of the LORAN signals presented by the antenna
can be very low, on the order of a few ten's of microvolts. To
prevent these signal from being totally obscured by noise by the
time they reach the receiver 12, a preamplifier 18 is provided to
increase the signal level before the LORAN signals are applied to
the transmission line 16.
The preamplifier 18 is preceded by a low frequency band pass filter
20, which allows only frequencies of about 100 kHz.+-.25 kHz to
pass, and matches the impedance of the antenna 10 to the input
impedance of the preamplifier. The band pass filter 20 thereby
provides for maximum transfer of LORAN signal power from the
antenna to the receiver, while preventing unwanted signals present
at the antenna, particularly the relatively high power VHF signals
produced by the transceiver 14 which would overload and possibly
damage the preamplifier 18, from reaching the input of the
preamplifier.
A low pass filter 22 is disposed between the output of the
preamplifier 18 and the transmission line 16. The low pass filter
allows the low frequency signals picked up by the antenna and
amplified by the preamplifier to be coupled to the transmission
line while preventing the relatively high power VHF signals
produced by the transceiver 14 from reaching the preamplifier
through its output circuitry. It also allows DC power to be
provided to the preamplifier, as is explained below.
VHF communications signals are coupled between the antenna 10 and
the transmission line 16 by a high frequency band pass filter 24.
The high frequency band pass filter permits signals of about 150
MHz.+-.2.5 MHz to pass from the antenna to the transmission line
with a good impedance match and vice versa, while preventing the
low frequency signals amplified by the preamplifier 18 from being
applied to the input of the low frequency band pass filter, which
would otherwise produce undesirable feedback. In addition, the high
frequency band pass filter minimizes the shunt capacitance applied
to the antenna 10 so as to minimize the attenuating affect that it
has on the low frequency signals picked up by the antenna, as is
explained below.
At the end of the transmission line 16 closest to the receiver 12
and the transceiver 14, a low pass filter 26 is provided to couple
the transmission line to the receiver and a high pass filter 28 is
provided to couple the transceiver to the transmission line. The
low pass filter 26 permits low frequency navigation signals to pass
from the transmission line to the input of the receiver 12 and DC
power to pass from the receiver to the transmission line, while
preventing relatively high power VHF signals from reaching, and
overloading, the LORAN receiver. The high pass filter 28 permits
the high frequency communication signals to pass between the
transmission line 16 and the VHF transceiver, while blocking the
low frequency signals and thereby preventing them from being
unnecessarily attenuated by the transceiver input.
Turning now to FIG. 2 the preamplifier 18 is a low-noise amplifier
presenting an input impedance to the low frequency band pass filter
20 of about 33 kohms and an output impedance to low pass filter 22
of about 50 ohms at approximately 100 kHz. In the preamplifier,
transistors 30, 32, and 34 are configured as a noninverting
operational amplifier whose alternating current ("AC") voltage gain
is determined by resistors 36 (4.7 kohms) and 38 (1 kohms).
Transistor 40 is provided to regulate the current in the
amplifier.
In the three-stage amplifier represented by transistors 30, 32, and
34, the operating point of each of those transistors is set by
resistors 42 (15 kohms), 44 (1 kohms) and 46 (47 ohms), together
with the current regulator circuitry. The voltage across resistor
46 is detected, divided, and applied to the base of transistor 40
by resistor 48 (27 kohms) and resistor 50 (15 kohms) so as to
regulate the current drawn by the amplifier to maintain a
substantially constant DC voltage across resistor 46. That is
accomplished by transistor 40, which reflects any change in voltage
as a change in current through resistor 52 (390 kohms), which
adjusts the voltage at the base of transistor 30 and, hence, the
voltage across resistor 46.
Although there is a resistor 54 (33 kohms) between resistor 52 and
the base of transistor 30, to set the input impedance of the
amplifier as is explained below, very little DC flows through
resistor 54. Similarly, very little DC flows through resistor 36.
Since the DC voltage drop across resistors 54 and 36 is essentially
constant, and since the base-emitter voltage of transistor 30 is
essentially constant, the change in voltage across resistor 52
caused by corresponding changes in the current through that
resistor, adjusts the voltage across resistor 46.
Capacitor 56 (47 nanofarads) acts as a low pass filter with respect
to the AC signal to be amplified; that is, it prevents any of the
AC signal from appearing across the base emitter junction of
transistor 40, which would be amplified and modulate the current
regulation signal. Similarly, capacitor 58 (47 nanofarads) acts as
a low pass filter; that is, it shunts to ground the AC signal that
would otherwise appear at the collector of the current regulator
transistor 40. Diode 62 is placed in the collector circuit of
transistor 40 to prevent the collector-base junction from becoming
reverse biased, as that would create AC instability of the current
regulation circuit. The current regulation circuit allows the
amplifier to operate in response to a DC supply voltage ranging
from about 3.5 volts up to about 20 volts while maintaining the
current operating point of the amplifier substantially
constant.
From an AC standpoint, the preamplifier 18 is coupled to the band
pass filter 20 by capacitor 60 (220 picofarads). It operates
essentially as a blocking capacitor and has minimal affect on the
transfer characteristics of the band pass filter 20 or the input
impedance of the preamplifier. The input impedance of the
preamplifier is, in fact, set by resistor 54. It is chosen so as to
maximize the signal-to-noise ratio at the input of the
preamplifier. To that end, it is selected to be substantially equal
to the ratio of the equivalent noise voltage divided by the
equivalent noise current at the input of the amplifier.
Diodes 64 and 66 are provided for limiting the amplitude of the
input signal to the preamplifier. While the low frequency
navigation signals which the preamplifier is intended to amplify
would not ordinarily exceed the limit established by these diodes,
other spurious signals may exceed that limit, so the diodes provide
protection against them.
The AC gain of the amplifier is substantially equal to 1 plus the
ratio of value of resistor 36 to the value of resistor 38. In this
case that is 5.7. This is because the amplifier is configured as an
operational amplifier; that is, the open loop gain of the
three-stage amplifier represented by transistors 30, 32, and 34 is
very high in relationship to the gain determined by resistors 36
and 38. Capacitor 68 (47 nanofarads) blocks DC, but allows AC to
flow to ground. The output impedance of the amplifier is set
primarily by resistor 46 and to some extent by the output impedance
of the three transistor operational amplifier. Together they
provide an output impedance of substantially 50 ohms, which matches
the impedance of the transmission line 16.
The low frequency band pass filter 20, connected to the antenna 10,
performs the functions of both matching the antenna impedance to
the input impedance of the preamplifier 18, and of preventing
unwanted signals present at the antenna from reaching the input of
the preamplifier. Such unwanted signals include signals above and
below the LORAN frequency which are picked up by the antenna, as
well as the relatively powerful high frequency signals supplied to
the antenna by the VHF transceiver
The impedance matching and filtering characteristics of the low
frequency band pass filter are best explained with respect to the
substantially equivalent circuit diagram shown in FIG. 3. In that
diagram the signal detected at the antenna is represented by
voltage generator 70 and the source impedance of the antenna is
represented by capacitor 72. The band pass circuit is a Butterworth
filter formed by the distributed capacitance of the antenna,
represented by capacitor 72, a series inductance 74, a shunt
capacitance 76, and a shunt inductance 78. The design of a
Butterworth filter is well understood by persons of ordinary skill
in the art.
In FIG. 2, the inductance 74, capacitance 76, and inductance 78 are
represented as discreet components by variable inductor 80
(nominally 119 millihenries), capacitor 82 (39 picofarads), and
variable inductor 84 (nominally 34 millihenries), respectively. The
variable inductors are used to adjust the band pass characteristics
of the filter. Resistor 86 (4.7 kohms) is employed in series with
inductor 80 to present a high input resistance to high frequency
signals present at the antenna and to any DC current that might be
applied to the antenna. The neon lamp 88 provides yet further
protection to the input of the preamplifier by limiting the input
voltage, particularly by limiting the maximum voltage that can be
applied to the input of the preamplifier in the face of large
voltage transients, such as lighting strikes and the like. It
should be recognized that the component values are representative
only and that the actual values used for a given embodiment of the
invention may be different depending on the distributed reactances
of the discrete components and the physical arrangement
thereof.
The low pass filter 22, which couples the output of the
preamplifier 18 to the transmission line 16, is a two pole low pass
filter comprising shunt capacitor 90 (180 picofarads) and series
inductor 92 (10 microhenries). This couples the approximately 50
ohm output impedance of the preamplifier 18 to the 50 ohm impedance
of the transmission line 16, permits low frequency navigation
signals to be applied by the preamplifier to the transmission line,
permits DC power to be applied from the transmission line to the
preamplifier, and blocks high frequency communication signals from
being coupled from the transmission line to the preamplifier
through the output stage of the preamplifier.
The high frequency band pass filter 24 is best explained first with
respect to FIG. 4. At 150 MHz, the antenna presents an essentially
resistive 50 ohm impedance represented by resistor 94, to which the
impedance of the filter is to be matched. However, to prevent
significant attenuation of received LORAN signals at the antenna,
the filter must present a relatively high impedance at 100 kHz.
This is accomplished by capacitance 96. Since the reactance of
capacitance 96 is unwanted at 150 MHz, inductance 98 is employed to
cancel it out. These two reactances make the filter a band pass
filter rather than a high pass filter.
Filter 24 utilizes distributed reactances as well as discreet
elements. Capacitance 96 and inductance 98 form a series resonant
circuit. Inductance 100 and the distributed capacitance 102
associated with the corresponding discreet inductor form a
parallel, shunt resonant circuit. Capacitance 104 and the
inductance in the leads of the corresponding discreet capacitance,
represented by inductance 106, form another series resonant
circuit. Together with the antenna 10, these reactances provide a
relatively flat frequency response and good impedance match to the
transmission line 16 over a frequency range of approximately 150
MHz.+-.2.5 MHz.
In FIG. 2, capacitance 96 is represented by capacitor 108 (3.9
picofarads), inductance 98 is represented by inductor 110 (280
nanohenries), inductance 100 and capacitance 102 are represented by
inductor 112 (10 nanohenries), and capacitance 104 and inductance
106 are represented by capacitor 114 (56 picofarads), respectively.
Since inductors 110 and 112 essentially present a short to ground
at the low frequency of 100 kHz, a relatively low value is needed
for capacitor 108, so as to minimize attenuation of low frequency
navigation signals detected by the antenna. Capacitor 114, together
with the rest of the components of the filter, prevents low
frequency signals amplified by the preamplifier from being fed back
to the antenna and then to the input of the preamplifier, and also
prevents DC power from being shorted to ground.
At the other end of the transmission line 16, which is terminated
at its ends by connectors 126 and 128, the low pass filter 26 for
coupling the transmission line to the LORAN receiver is a single
pole low pass filter comprising inductor 116 (3.9 microhenries) and
capacitor 118 (82 picofarads). It is connected to the receiver by
connector 130. The high pass filter 28 coupling the transmission
line to the VHF transceiver comprises capacitor 120 (56 picofarads)
which, together with its lead inductance, is a single stage band
pass filter. Capacitor 120 blocks DC power provided by the LORAN
receiver to the preamplifier and low frequency signals present on
the transmission line 16 from reaching the antenna connection of
the VHF transceiver. The high pass filter is connected to the VHF
transceiver by connector 132.
An alternative embodiment of the invention is shown in FIG. 5. In
this case, the general configuration is the same as in the
aforedescribed embodiment except that the antenna 10 is connected
to the low frequency band pass filter 20 and to the high frequency
band pass filter 24 by a short length of transmission line 122,
typically about 4.5 inches long and having a characteristic
impedance of 75 ohms. Such an embodiment is used, for example,
where it is undesirable to have any portion of the antenna system
except for the antenna itself protruding from the surface of the
vehicle. This requires that the filters and preamplifier be placed
beneath the surface of the vehicle, which requires that a short
length of transmission line be disposed between the base of the
antenna and the input to the system.
The effect of the transmission line 122 depends upon various
factors, primarily the length of the transmission line 122, the
length of the antenna 10, the signal frequency, and the
characteristic impedance of the transmission line 122. But in the
example given, the distributed shunt capacitance of the antenna is
increased at low frequencies and the antenna has a predominately
inductive reactance characteristic at high frequencies. As shown in
FIG. 6, which represents the equivalent circuit of the antenna and
short transmission line at high frequencies, the resistance 123
represents the resistive component of the antenna and the
inductance 124 represents the source reactance of the antenna at
VHF frequencies.
To compensate for the increased distributed shunt capacitance of
the antenna 10 and transmission line 122, the values of the
discreet components of the low frequency band pass filter are
changed. As shown in FIG. 5, a different inductor 130 (73
millihenries), capacitor 132 (82 picofarads), and inductor 134 (17
millihenries) are provided. Because the transmission line 122
causes the voltage of the transmitted VHF signal to be relatively
high at the input to the low frequency band pass filter 20,
inductor 126 is added to protect the rest of the filter and the
input to the preamplifier. Inductor 126, together with its
distributed capacitance, acts as a choke at 150 MHz, which
substantially prevents the VHF signal from passing to the rest of
the low frequency input circuit. It has been found that these
components will provide an appropriate match of the impedance at
the end of the transmission line 122 to the input impedance of the
preamplifier 18, though it is to be understood that in a given
embodiment the actual values will depend upon the distributed
reactances in the circuit.
Similarly, to compensate for the reactance characteristic of the
combined antenna 10 and transmission line 122 at high frequencies
the values of the components in the high frequency band pass filter
are changed. Indeed, in this embodiment the high frequency band
pass filter also has as an important function matching the
impedance at the end of the transmission line 122 to the impedance
of transmission line 16. Thus, as shown in FIG. 5, a different
inductor 136 (358 nonohenries), inductor 138 (338 nanohenries), and
capacitor 140 (2-14 picofarads, variable) are provided. It has been
found that these components provide a proper match of the impedance
at the end of the transmission line 122 to the 50 ohm impedance of
the transmission line 16.
Although the embodiments shown in the foregoing description are
directed specifically to the simultaneous reception of LORAN-C
signals at about 100 kHz and the transmission or reception of VHF
communication signals at about 150 MHz, it is to be recognized that
other frequencies may be used for other purposes, without departing
from the principles of this invention. In addition, while specific
component values have been given in this description, it is to be
recognized that other component values might be used, particularly
for different frequency bands, without departing from the
principles of this invention.
The terms and expressions which have been employed in the foregoing
specification are used herein as terms of description and not of
limitation, and there is no intention, in the use of such terms and
expressions, of excluding equivalents of the features shown and
described or portions thereof, it being recognized that the scope
of the invention is defined and limited only by the claims which
follow.
* * * * *